Landfills produce a large quantity of methane, a greenhouse gas many consider more harmful than carbon dioxide.

Landfills pose a contamination risk to groundwater.

Space is becoming more scarce for new landfills.

At Walmart, zero waste is so important that we made it one of our three guiding global sustainability goals. For us, zero waste means just that—nothing going to the landfill. When we made our waste commitment in 2005, we had no idea how we would get there, and while we're much closer today than we were six years ago, we still don't know how we are going to get to zero.

The biggest challenge we faced when we started was the lack of non-landfill alternatives for waste. Walmart operates 4,400 stores, Sam's Club locations and distribution centers across the country, so a one-size-fits-all approach will not work. If you think about it, the solution for organic waste in California won't work in Michigan, and what works in Michigan won't work in Florida. We needed to create a nationwide infrastructure to handle the diverse waste streams from our stores. We have roughly 55 waste categories including:

car batteries, automotive oil and tires

prescription bottles

plastic bags and bottles

produce and meat

floral and bakery icing buckets

apparel

We've created a full-scale waste reduction program in California that is keeping 81 percent of the waste we generate out of landfills. We have now implemented the program in every state and are hopeful that we will see similar results. We are proud of this progress, but realize we still have more work to do. Our program consists of three main components:

Donating healthy, nutritious food to food banks around the country. In 2010, Walmart donated 256 million pounds of food to hunger relief organizations—the equivalent of 197 million meals.

Recycling cardboard, paper, aluminum, plastic bags and roughly 30 other items through the super sandwich bale (SSB) program. Items not eligible for the SSB, including wood pallets, polystyrene plastic and apparel, are sent to Walmart's return centers for reuse or recycling.

Creating animal feed, energy or compost from food not eligible for donation and other organic products following the EPA's food waste hierarchy. We now provide meat to 130 wild animal parks around the country to help them feed their tigers, lions and other large animals.

One of my favorite examples is what has been accomplished in our Tire and Lube Express (TLE). In the TLEs, we change oil, tires and batteries for our customers. The Walmart TLE team is closing in on zero waste. We recycle used oil, tires, wheel weights, oil filters, and batteries. Because zero waste means nothing goes to the landfill, we are actively looking for a non-landfill solution for the other waste streams in our TLE, including oil bottles and windshield wipers.

So you're probably asking what makes up our final 20 percent. It's mostly waste from our restrooms, parking lots, leased spaces (restaurants, banks, hair and nail salons) and contaminants, such as used meat packaging. At this point we haven't found any alternatives that are as effective, scalable and environmentally preferable to the landfill for this waste. I'm hopeful the TreeHugger audience will have some great ideas for what can be done with it. While completely eliminating waste is a daunting challenge, we are determined to continue working to find solutions that will benefit the environment, communities as well as other businesses.